An electrical fitting is usually required at installations where a flexible electrical cable must pass through an opening in a hard surface, such as into an enclosure, through a bulkhead, or into a rigid conduit. The electrical fitting is intended to form a firm mechanical grip on the cable to prevent it from sliding in or out of the opening through which it passes. Typically, it also reduces strain on the cord and on any connections within the fixture.
Most strain relief electrical fittings on the market today consist of a threaded body, a deformable bushing, a slip ring, and a threaded gland. The deformable bushing compresses around the outer jacket of an electrical cable as the threaded body and threaded gland on either side of the deformable bushing are screwed together. As the bushing is compressed in an axial direction, its inner diameter decreases, causing the bushing to push inwardly on the electrical cable. If the electrical fitting is tightened sufficiently, the pressure of the bushing on the cable creates enough friction force to prevent the cable from sliding through the fitting. However, if the threaded gland is not tightened onto the threaded body sufficiently, the cable can be pulled loose, potentially causing an electrical short.
Other known bushings use extruded solid gripping teeth between the cable and the bushing to produce a more aggressive wedge action. However, the solid gripping teeth can cause damage to the protective outer jacket of the cable.